Disc playback apparatus and method

ABSTRACT

A disc playback apparatus is configured to reproduce information by demodulating and decoding a radio-frequency signal obtained by receiving reflected light of a beam applied to a recording surface of a removable disc when the disc is loaded in the disc playback apparatus. The apparatus includes a disc format determining unit adapted to determine the format of the loaded disc, a high-frequency compensation unit adapted to compensate for a high-frequency component of the radio-frequency signal, a low-frequency compensation unit adapted to compensate for a low-frequency component of the radio-frequency signal, and a control unit adapted to determine which of the high-frequency compensation unit or the low-frequency compensation unit is to be used to compensate for a frequency characteristic of the radio-frequency signal based on the determined disc format.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese PatentApplication JP 2004-278818 filed in the Japanese Patent Office on Sep.27, 2004, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a disc playback apparatus and methodfor reproducing information recorded on a disc-shaped recording medium,and, particularly, to a disc playback apparatus, e.g., a compact disc(CD) drive, and method for reproducing information based on aradio-frequency (RF) signal obtained by receiving reflected light of alight beam applied to a recording surface of a recording medium.

More specifically, the present invention relates to a disc playbackapparatus and method for reproducing information from a recordingsurface of removable discs of different types that are removably loadedin the disc playback apparatus, such as a CD-recordable (CD-R) disc anda CD-rewritable (CD-RW) disc. In particular, the present inventionrelates to a disc playback apparatus and method for reproducinginformation without errors even if the type of disc loaded in the discplayback apparatus changes.

2. Description of the Related Art

With the recent technological innovations, various types of calculationsystems have been developed and become commercially available. Ingeneral, calculation systems include an external storage system forstoring necessary information or installing an operating program.

One typical storage device is a magnetic recording hard disk drive.Several magnetic media are accommodated as recording media in the driveunit, and each medium is rotated at a high rate by a spindle motor. Eachmedium is coated with a magnetic material that is plated with nickelphosphorous or the like. Scanning of a magnetic head on a surface of arotating medium along the radius of the rotating medium causesmagnetization on the medium to write or read data.

The use of hard disks has already become widespread. For example, harddisks are used as standard external storage devices for personalcomputers, and are used for installing various software programs, suchas operating system (OS) programs necessary for booting the computersand utility application programs, or storing created or edited files.

Another type of external storage device is configured such that aremovable recording disc for transferring an operation program to beinstalled on a hard disk or data content among a plurality of systems isremovably loaded into the device to reproduce information. In the past,portable media, such as flexible discs, were the mainstream. Recently,with the increasing size of programs and data content, disc-shapedrecording media with a relatively large capacity, such as CDs, andapparatuses for playing back such discs have increasingly becomepopular.

A CD is a recording medium from which information can be reproducedbased on an RF signal obtained by receiving reflected light of a lightbeam applied to a recording surface of the disc. Recently, not onlyread-only CDs (“press CDs”) in which information is recorded as pits bymanufacturers, such as compact disc read-only memory (CD-ROM) discs, butalso discs on which information is recordable by end users, such as CD-Rand CD-RW discs, have become available, and drives compatible with suchuser-recordable discs have increasingly become popular.

There are a variety of CDs depending on the combination of thereflective layer and the dye of the recording layer (see, for example,Yasuhiro MORI, “CD-R/DVD-R Masuta (CD-R/DVD-R Master)”, Sansai Mook,pages 90 to 92, Jun. 1, 2002). For example, a CD-R disc is composed of asubstrate, an organic dye layer, a recording layer, a reflective layer,an overcoat layer, and a protective layer. The recording layer is formedwith dyes, e.g., cyanine blue dye, phthalocyanine dye, or azo dye,depending on the disc manufacturer. The reflective layer is one of twocolors, either gold or silver. Data is written on a CD-R disc bycreating pits with a semiconductor laser beam with a wavelength of 780nanometers. Specifically, the dye decomposes with heat of the laserbeam, and the substrate of polycarbonate is exposed to heat and isgrooved to produce pits. Once data has been recorded, the recorded datais not erasable. A press CD has a similar structure to a CD-R disc, buthas no dye layer because the reflective layer is directly deposited byaluminum evaporation on a polycarbonate substrate that is preformed withlands and grooves.

A CD-RW disc is composed of a substrate, a reflective layer, a lowerprotective layer, a phase-change recording layer, a reflective layer, anupper protective layer, an ultraviolet (UV) coating layer, and aprinting surface. The phase-change recording layer is rapidly heatedwith the energy delivered by a laser beam, and is then rapidly cooled toproduce amorphous areas, thereby forming pits. These pits reflect lesslight than the remaining crystalline areas. Erasing is performed byrapidly heating the recording layer with the energy delivered by thelaser beam, which is of a temperature between those for recording andplayback, and, then, slowly cooling it to return the amorphous stateback to the crystalline state.

As discussed previously, a recording surface of such CDs is irradiatedwith a laser beam, and the light reflected from the recording surface isreceived to produce an RF signal, based on which information isreproduced. When the RF signal is not reproduced under good conditions,it is difficult to perform high-quality digital signal processing.

A CD player designed for music reproduction or a CD-ROM player designedfor computer-data reproduction is not acceptable as a system when dataerrors occur in excess of a predetermined amount. Data errors arecorrectable by a signal processing circuit or the like. However, if theRF signal reproduced from a disc does not have sufficiently highquality, errors are not fully corrected for by the correction ability ofthe signal processing circuit, and data errors occur.

When a disc of the CD or CD-ROM format is played back, errors may not befully corrected for due to low quality of the RF reproduction signalwaveform (the eye pattern for 3T to 11T), and the reproduced sound maybe contaminated with noise or data errors may occur. It is thereforenecessary to perform frequency correction depending on the reproduced RFsignal to facilitate digital signal processing.

In order to improve the quality of an RF reproduction signal of a discof the CD or CD-ROM format, high-frequency compensation is generallyperformed on the RF reproduction signal. An RFhigh-frequency-compensation equalizer circuit or the like is typicallyused as a compensator at the stage before performing signal processingon a low-quality RF reproduction signal that is produced by acombination of the disc and an optical pickup, etc.

There has been proposed, for example, an optical disc device includingan equalizer for changing a gain-frequency characteristic of an RFsignal depending on the received light level of the light reflected froma disc that is irradiated with a laser beam, in which the characteristicof the equalizer is changed depending on the received light level,thereby reproducing information without errors irrespective of the disctype (see, for example, Japanese Unexamined Patent ApplicationPublication No. 2001-14680, FIG. 2). This optical disc device controlsthe rise characteristic of the RF signal so as to be constant byincreasing the gain in the high-frequency band when the received lightlevel is low and by decreasing the high-frequency gain when the receivedlight level is high.

However, due to the different natures from one CD format to another, thefrequency band in which the RF reproduction signal waveform is degradedalso differs. The received light level of the reflected light, i.e., thereproduced RF signal, differs depending on the disc type. Nevertheless,it is necessary to ensure the compatibility of the disc playbackapparatus with each format of disc.

In high-frequency compensation, the frequency and the amount ofcompensation may be set to one fixed value or may be set to a pluralityof or continuously variable values. In either case, high-frequencycompensation is mostly effective for press CDs or CD-R discs susceptibleto level degradation of a high-frequency signal of, e.g., 3T to 4T.

On the other hand, high-frequency compensation may often bedisadvantageous for CD-RW discs (of the recordable erasable type)susceptible to level degradation of a low-frequency signal of, e.g., 10Tto 11T. It is therefore difficult to improve the quality of an RFreproduction signal of a CD-RW disc using only an RF equalizer of therelated art designed only for high-frequency compensation.

Accordingly, a high-frequency compensation circuit of the related artmay be effective for press CDs or CD-R discs, but may often beineffective for recordable erasable CD-RW discs.

SUMMARY OF THE INVENTION

It is therefore desirable to provide a high-performance disc playbackapparatus, e.g., a CD drive, and method for suitably reproducinginformation based on an RF signal obtained by receiving reflected lightof a light beam applied to a recording surface of a recording medium.

It is also desirable to provide a high-performance disc playbackapparatus and method for suitably reproducing information from arecording surface of removable discs of different types that areremovably loaded in the disc playback apparatus, such as a CD-R disc anda CD-RW disc.

It is further desirable to provide a high-performance disc playbackapparatus and method for reproducing information without errors even ifthe type of the disc loaded in the disc playback apparatus changes.

It is still further desirable to provide a high-performance discplayback apparatus and method for reproducing information without errorsby performing appropriate frequency compensation to improve the qualityof the RF reproduction signal even if the type of the disc loaded in thedisc playback apparatus changes.

A disc playback apparatus according to an embodiment of the presentinvention reproduces information by demodulating and decoding aradio-frequency signal obtained by receiving reflected light of a beamapplied to a recording surface of a removable disc when the removabledisc is loaded in the disc playback apparatus. The disc playbackapparatus includes disc format determining means for determining theformat of the loaded disc, a high-frequency compensation unit adapted tocompensate for a high-frequency component of the radio-frequency signal,a low-frequency compensation unit adapted to compensate for alow-frequency component of the radio-frequency signal, and a controlunit adapted to determine which of the high-frequency compensation unitor the low-frequency compensation unit is to be used to compensate for afrequency characteristic of the radio-frequency signal based on thedetermined disc format.

The disc format determining means may determine the format of the discbased on, for example, but not limited to, an intensity of the reflectedlight from the recording surface of the loaded disc.

According to an embodiment of the present invention, the disc playbackapparatus is compatible with discs of multiple CD formats. The controlunit may determine that the high-frequency compensation unit is to beused to compensate for the frequency characteristic of theradio-frequency signal when a read only memory compact disc (e.g., apress CD) or a recordable compact disc (e.g., a CD-R disc) is loaded inthe disc playback apparatus, and may determine that the low-frequencycompensation unit is to be used to compensate for the frequencycharacteristic of the radio-frequency signal when a recordable erasablecompact disc (e.g., a CD-RW disc) is loaded in the disc playbackapparatus.

According to an embodiment of the present invention, the disc playbackapparatus may further include signal quality determining means fordetermining whether or not the quality of the radio-frequency signal isdegraded. For example, the signal quality determining means may countthe number of C1 errors in the demodulated and decoded radio-frequencysignal, and may determine the quality of the radio-frequency signalbased on the counted C1 errors. In this case, the control unit maycompensate for the frequency characteristic of the radio-frequencysignal using the high-frequency compensation unit or the low-frequencycompensation unit in response to a degradation in the quality of theradio-frequency signal that is determined by the signal qualitydetermining means. The C1 error count is also referred to as a “blockerror rate (BLER),” and is represented as the number of errors correctedin 7350 blocks into which data for one second is divided, which isexpressed in count per second (cps).

An embodiment of the present invention pertains to a disc playbackapparatus for reproducing information based on an RF signal obtained byreceiving reflected light of a laser beam applied to a recording surfaceof a disc. It is difficult for such a disc playback apparatus to performhigh-quality digital processing if the RF signal is not reproduced undergood conditions. It is therefore necessary to perform frequencycorrection depending on the reproduced RF signal to facilitate digitalsignal processing.

In order to improve the quality of an RF reproduction signal of a discof the CD or CD-ROM format, high-frequency compensation is generallyperformed on the RF reproduction signal. In high-frequency compensation,the frequency and the amount of compensation may be set to one fixedvalue or may be set to a plurality of or continuously variable values.However, a problem arises in that the received light level of thereflected light, i.e., the reproduced RF signal, differs from one formatto another and the frequency band in which the RF reproduction waveformis degraded differs.

High-frequency compensation is effective for press CDs or CD-R discssusceptible to level degradation of a high-frequency signal of, e.g., 3Tto 4T, but may often be disadvantageous for CD-RW discs (of therecordable erasable type) susceptible to level degradation of alow-frequency signal of, e.g., 10T to 11T. It is therefore difficult toimprove the quality of the RF reproduction signal of a CD-RW disc usingonly an RF equalizer designed only for high-frequency compensation.

According to an embodiment of the present invention, therefore, in orderto support insufficient quality of the RF reproduction signal, it isdetermined whether the disc is a press CD or a CD-R disc, or a CD-RWdisc before actually transmitting a reproduction signal. If the disc isa press CD or a CD-R disc, high-frequency compensation is performed; ifthe disc is a CD-RW disc, low-frequency compensation is performed,thereby improving the reproduction quality of the RF signal.

When the high-frequency signal of the PF reproduction waveform isdegraded due to bad recording conditions of a CD-R disc, environmentalreasons, etc., high-frequency compensation for increasing thehigh-frequency component of the RF waveform compared to thelow-frequency component is performed, thereby achieving an improvementwithout errors.

When the low-frequency signal of the RF reproduction waveform isdegraded due to bad recording conditions of a CD-RW disc, environmentalreasons, etc., low-frequency compensation for increasing thelow-frequency component compared to the high-frequency component greatlyimproves the signal errors.

A conceivable mechanism for implementing the compensation functiondepending on the format of the medium is an automatic correctionmechanism for determining the C1 error count value, the RF jitter value,or the like and performing the compensation function in excess of apredetermined reference value to implement the compensation function.

According to an embodiment of the present invention, a high-performancedisc playback apparatus and method for suitably reproducing informationfrom a recording surface of removable discs of different types that areremovably loaded in the disc playback apparatus, such as a CD-R disc anda CD-RW disc, can be achieved.

According to another embodiment of the present invention, ahigh-performance disc playback apparatus and method for reproducinginformation without errors even if the type of the disc loaded in thedisc playback apparatus changes can be achieved.

According to another embodiment of the present invention, ahigh-performance disc playback apparatus and method for reproducinginformation without errors by performing appropriate frequencycompensation to improve the quality of the RF reproduction signal evenif the type of the disc loaded in the disc playback apparatus changescan be achieved.

A disk playback apparatus according to an embodiment of the presentinvention performs high-frequency compensation when the loaded disc is apress CD or a CD-R disc susceptible to level degradation of ahigh-frequency signal, and performs low-frequency compensation when theloaded disc is a CD-RW disc (of the recordable erasable type)susceptible to level degradation of a low-frequency signal, therebyimproving the quality of the RF reproduction signal. The occurrence ofnoise for music reproduction and the occurrence of data errors for datareproduction are therefore prevented. If the quality of the RFreproduction signal is low, compensation is performed depending on themedia format, thereby achieving high-quality reproduction.

Further objects, features, and advantages of the present invention willbecome apparent from the following detailed description of an embodimentof the present invention, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the hardware configuration of adisc playback apparatus according to an embodiment of the presentinvention;

FIG. 2 is a schematic internal block diagram of an equalizer in the discplayback apparatus shown in FIG. 1;

FIG. 3 is a flowchart showing a data reproducing process performed bythe disc playback apparatus;

FIG. 4 is a flowchart showing the data reproducing process performed bythe disc playback apparatus;

FIG. 5 is a circuit diagram of a high-frequency compensation unit in theequalizer;

FIG. 6 is a gain-frequency characteristic chart of the high-frequencycompensation unit shown in FIG. 5;

FIG. 7 is a diagram showing normal RF reproduction waveforms of a CD-Rdisc;

FIG. 8 is a diagram showing RF reproduction waveforms with thehigh-frequency signal degraded due to bad recording conditions of theCD-R disc, environmental reasons, etc.;

FIG. 9 is a diagram showing RF reproduction waveforms obtained byperforming high-frequency compensation on the RF waveforms shown in FIG.8 using the high-frequency compensation unit shown in FIG. 5;

FIG. 10 is a circuit diagram of a low-frequency compensation unit in theequalizer;

FIG. 11 is a gain-frequency characteristic chart of the low-frequencycompensation unit shown in FIG. 10;

FIG. 12 is a diagram showing normal RF reproduction waveforms of a CD-RWdisc;

FIG. 13 is a diagram showing RF reproduction waveforms with thelow-frequency signal degraded due to bad recording conditions of theCD-RW disc, environmental reasons, etc.;

FIG. 14 is a diagram showing RF reproduction waveforms obtained byperforming high-frequency compensation on the RF waveforms shown in FIG.13 using the high-frequency compensation unit shown in FIG. 5; and

FIG. 15 is a diagram showing RF reproduction waveforms obtained byperforming low-frequency compensation on the RF waveforms shown in FIG.13 using the low-frequency compensation unit shown in FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described in detail withreference to the drawings.

FIG. 1 schematically shows the hardware configuration of a disc playbackapparatus according to an embodiment of the present invention.

The disc playback apparatus shown in FIG. 1 is capable of reading datafrom discs of multiple CD formats, such as CD-ROM (press CD), CD-R, andCD-RW discs, based on a change in the intensity of the light reflectedfrom a pattern of pitted and unpitted areas on a recording surface ofthe discs that is irradiated with a laser beam. Such optical discs arespecified so as to have a minimum data length (minimum pit length) of 3Tand a maximum data length (maximum pit length) of 11T, and record datathat is modulated using EFM (Eight to Fourteen Modulation) that belongsto the class of DC-free codes.

In the disc playback apparatus, a central processing unit (CPU) 1 isconnected to a bus 2, and the bus 2 is also connected to a random accessmemory (RAM) 3 serving as a work memory, a read-only memory (ROM) 4serving as a non-volatile memory storing program data 4 a for systemcontrol and parameter data 4 b for control, a servo signal processingcircuit 5, a spindle motor 6 for rotating an optical disc 7 via theservo signal processing circuit 5, and an optical pickup 8. An equalizer11, a digital signal processing circuit 12, a decoder 13, and a hostinterface circuit 14 communicating with a host computer 18 are furtherconnected via an input/output port 10. The equalizer 11 equalizes thewaveform of an RF signal from an amplification circuit 9, and thedigital signal processing circuit 12 binarizes the output of theequalizer 11, and then generates a clock. The digital signal processingcircuit 12 reproduces data based on the generated clock, and performserror correction on the reproduced data. The decoder 13 performsprocessing, such as error correction, on the output of the digitalsignal processing circuit 12, and decodes the data into the originaldata sequence.

The host interface circuit 14 complies with the interface standard, suchas USB (Universal Serial Bus) or SCSI (Small Computer System Interface).The disc playback apparatus is connected to a host system, such as apersonal computer, via the host interface circuit 14 and an external bus17.

The servo signal processing circuit 5 synchronously controls the spindlemotor 6 to rotate and the optical pickup 8 to scan in the radialdirection of the disc, and carries out the focusing and trackingoperations.

When the disc playback apparatus is powered on, the program data 4 astored in the ROM 4 is loaded into a main memory of the CPU 1, and theCPU 1 therefore functions as a RAM controller 15 and a system controller16, as indicated by a broken-line block in FIG. 1.

The optical discs 7 compatible with the disc playback apparatus includeCD-ROM, CD-R, and CD-RW discs. The disc playback apparatus is capable ofnormal-speed or x-time-speed playback in response to an instruction fromthe host computer 18.

The equalizer 11 is provided for the purpose of correcting fordeterioration of the eye pattern based on a modulation transfer function(MTF). In playing back the optical disc 7, the impulse response waveformoutput from the optical pickup 8 has spreading tails with the eyepattern being flattened at the center due to the disc moldingconditions, etc., thus preventing binarization with respect to thecenter of the eye pattern from being stably performed. For example, theequalizer 11 increases the level of the RF reproduction waveform in thehigh-frequency region, thereby allowing the digital signal processingcircuit 12 to stably perform binarization and, equivalently, correctingfor the MTF degradation. The details of the frequency-band compensationability of the equalizer 11 are discussed below.

When the optical disc 7 is loaded in the disc playback apparatus, thespindle motor 6 starts rotating, and the optical disc 7 is rotated at aconstant linear velocity. When the rotation speed of the optical disc 7becomes constant, the optical pickup 8 irradiates a laser beam onto arecording surface of the optical disc 7 and reads the reflected light ofthe laser beam. The intensity of the reflected light changes dependingon the pattern of pitted and unpitted areas on the recording surface,and data is therefore read from the optical disc 7.

A CD-ROM or CD-R disc onto which data is written by changing the dyecolor has a high reflectivity, and a CD-RW disc onto which data iswritten by phase change has a low reflectivity (a CD-R disc reflectsabout four times as much light as a CD-RW disc). The disc reflectivitydetected by the optical pickup 8 is sent to the CPU 1 via the servosignal processing circuit 5 and the bus 2, and the CD format isdetermined.

The data read by the optical pickup 8 is supplied as a reproduced RFsignal to the digital signal processing circuit 12 via the amplificationcircuit 9 and the equalizer 11, and is then reproduced as digital databy the digital signal processing circuit 12. The reproduced digital datais supplied to the decoder 13, and is corrected for errors and convertedinto the original data sequence by the decoder 13.

Next, the operation of the equalizer 11 to compensate an RF reproductionwaveform will be discussed.

A CD player designed for music reproduction or a CD-ROM player designedfor computer-data reproduction is not acceptable as a system when dataerrors occur in excess of a predetermined amount.

A digital signal processing circuit or the like has a capability ofcorrecting for data errors. However, if the RF signal reproduced from adisc does not have sufficiently high quality, errors are not correctedfor even by the correction ability of the digital signal processingcircuit, and data errors occur. It is therefore necessary to performfrequency correction depending on the reproduced RF signal to facilitatedigital signal processing.

In order to improve the quality of an RF reproduction signal of a discof the CD or CD-ROM format, high-frequency compensation is generallyperformed on the RF reproduction signal. However, a problem arises inthat the received light level of the reflected light, i.e., thereproduced RF signal, differs from one format to another and thefrequency band in which the RF reproduction waveform is degradeddiffers.

High-frequency compensation is effective for press CDs or CD-R discssusceptible to level degradation of a high-frequency signal of, e.g., 3Tto 4T, but may often be disadvantageous for CD-RW discs (of therecordable erasable type) susceptible to level degradation of alow-frequency signal of, e.g., 10T to 11T. It is therefore difficult toimprove the quality of the RF reproduction signal of a CD-RW disc usingonly an RF equalizer designed only for high-frequency compensation.

According to the present embodiment, therefore, the equalizer 11includes a high-frequency compensation circuit for compensating fordegradation of the high-frequency component of the RF reproductionwaveform, and a low-frequency compensation circuit for compensating fordegradation of the low-frequency component. Degradation of the RFreproduction waveform is compensated for selectively using one of thecompensation circuits depending on the format of the loaded disc.Specifically, in order to support insufficient quality of the RFreproduction signal, it is determined whether the disc is a press CD ora CD-R disc, or a CD-RW disc before actually transmitting a reproductionsignal. If the disc is a press CD or a CD-R disc, high-frequencycompensation is performed; if the disc is a CD-RW disc, low-frequencycompensation is performed, thereby improving the reproduction quality ofthe RF signal.

FIG. 2 shows the internal structure of the equalizer 11 according to thepresent embodiment. As shown in FIG. 2, the equalizer 11 includes ahigh-frequency compensation unit 11-a adapted to perform high-frequencycompensation on an RF signal read by the optical pickup 8, anon-compensation unit 11-b adapted to transmit the RF signal withoutperforming compensation, a low-frequency compensation unit 11-c adaptedto perform low-frequency compensation on the RF signal, and a pair ofswitches 11-d and 11-e adapted to selectively switch an input terminalfrom which the RF signal is input to the equalizer 11 and an outputterminal through which the signal is output to the digital signalprocessor 12 from the equalizer 11 to any of the compensation units 11-ato 11-c.

In the initial state, the switches 11-d and 11-e connect thenon-compensation unit 11-b to the input terminal and the outputterminal. Switching of the switches 11-d and 11-e, that is, selection ofthe compensation units 11-a to 11-c, is performed by, for example, theCPU 1. The CPU 1 determines the quality of the RF reproduction signalbased on the decoded data or the like. If degradation in the quality isfound, the CPU 1 determines that it is necessary for the equalizer 11 toperform signal compensation.

The CPU 1 further determines the CD format based on the discreflectivity that is detected by the optical pickup 8 or the like. ACD-ROM or CD-R disc onto which data is written by changing the dye colorhas a high reflectivity, and a CD-RW disc onto which data is written byphase change has a low reflectivity (a CD-R disc reflects about fourtimes as much light as a CD-RW disc). Based on the disc reflectivitydetected by the optical pickup 8, the CD format of the loaded disc isdetermined.

If the determined CD format is the press CD or CD-R format when it isdetermined that compensation for the RF reproduction signal isnecessary, the CPU 1 changes the switches 11-d and 11-e to thehigh-frequency compensation unit 11-a to perform high-frequencycompensation, thereby improving the reproduction quality of the RFsignal. If the determined CD format is the CD-RW format when it isdetermined that compensation for the RF reproduction signal isnecessary, the CPU 1 changes the switches 11-d and 11-e to thelow-frequency compensation unit 11-c to perform low-frequencycompensation, thereby improving the reproduction quality of the RFsignal.

The digital signal processor 12 EFM-modulates the RF reproduction signalto reproduce digital data. The reproduced digital data is supplied tothe decoder 13, and is corrected for errors and converted into theoriginal data sequence by the decoder 13. After performing errorcorrection, the number of C1 errors is counted. The C1 error count isalso referred to as a “block error rate (BLER),” and is represented asthe number of errors corrected in 7350 blocks into which data for onesecond is divided, which is expressed in count per second (cps). Basedon the C1 error count, the CPU 1 determines the quality of the RFreproduction signal and the necessity of compensation for the RF signal.

FIGS. 3 and 4 are flowcharts showing a routine for a data reproducingprocess performed by the disc playback apparatus according to thepresent embodiment. The data reproducing process will be discussed inthe context of compensation for an RF reproduction signal.

This routine starts when a disc with a certain CD format is loaded intothe disc playback apparatus and when a instruction to start discplayback is input from the host computer 18.

First, it is determined whether the CD format of the loaded disc is thepress CD or CD-R format or the CD-RW format based on an intensity of thereflected light from a disc surface that is received by the opticalpickup 8 (step S1).

In the initial state, the switches 11-d and 11-e in the equalizer 11connect the non-compensation unit 11-b to the input terminal and theoutput terminal (step S2). The reproduction process is initiated in thisstate (step S3). The RF reproduction signal is EFM-modulated and is thusreproduced as digital data.

The reproduced digital data is supplied to the decoder 13, and iscorrected for errors and converted into the original data sequence bythe decoder 13. After performing error correction, the number of C1errors is counted (step S4). The decoder 13 stores the error count value(step S5).

When the C1 error count is reported from the decoder 13, the CPU 1determines whether or not the C1 error count is higher than apredetermined value (step S6). As long as the C1 error count is nothigher than the predetermined value, the data reproduction process iscontinuously performed without correcting the RF reproduction signal bythe equalizer 11 (step S7).

If the C1 error count is higher than the predetermined value in step S6,the CPU 1 checks the CD format of the currently loaded disc (step S8).

If the CD format of the disc is the CD-RW format, the CPU 1 changes theswitches 11-d and 11-e to the low-frequency compensation unit 11-c toperform low-frequency compensation (step S9).

Then, the number of C1 errors counted in step S5 is compared with thenumber of C1 errors counted after performing low-frequency compensation(step S13). In step S14, it is determined whether or not the number ofC1 errors is lower than the value counted before performinglow-frequency compensation.

If the C1 errors are not corrected and the quality of the RFreproduction signal is not improved, the CPU 1 changes the switches 11-dand 11-e to the non-compensation unit 11-b (step S16). Then, the routinereturns to step S2, and the reproduction process is continuouslyperformed without compensation of the RF reproduction signal by theequalizer 11.

If the C1 errors are corrected and the quality of the RF reproductionsignal is improved, the reproduction process is continuously performedwhile connecting the switches 11-d and 11-e to the low-frequencycompensation unit 11-c and performing low-frequency compensation (stepS15).

If the C1 error count is higher than the predetermined value (step S6)and the CD format is not the CD-RW format, i.e., the press CD or CD-Rformat (step S8), the CPU 1 changes the switches 11-d and 11-e to thehigh-frequency compensation unit 11-a to perform high-frequencycompensation (step S11).

Then, the number of C1 errors counted in step S5 is compared with thenumber of C1 errors counted after performing high-frequency compensation(step S17). In step S18, it is determined whether or not the number ofC1 errors is lower than the value counted before performinghigh-frequency compensation.

If the C1 errors are not corrected and the quality of the RFreproduction signal is not improved, the CPU 1 changes the switches 11-dand 11-e to the non-compensation unit 11-b (step S20). Then, the routinereturns to step S2, and the reproduction process is continuouslyperformed without compensation of the RF reproduction signal by theequalizer 11.

If the C1 errors are corrected and the quality of the RF reproductionsignal is improved, the reproduction process is continuously performedwhile connecting the switches 11-d and 11-e to the high-frequencycompensation unit 11-a and performing high-frequency compensation (stepS19).

Therefore, a system for automatically performing the optimumcompensation by determining the conditions of an RF signal reproducedfrom a disc at an appropriate time according to the above-describedprocess routine can be achieved.

FIG. 5 shows an example circuit structure of the high-frequencycompensation unit 11-a. In the example shown in FIG. 5, thehigh-frequency compensation unit 11-a is configured using a common-baseamplifier. FIG. 6 shows the gain-frequency characteristic of thehigh-frequency compensation unit 11-a. As can be seen from FIG. 6, thegain in the high-frequency band, or the high-frequency gain, is higherthan that in the low-frequency band. Thus, the level degradation of thehigh-frequency signal of, e.g., 3T to 4T, is compensated for.

FIG. 7 shows normal RF reproduction waveforms of a CD-R disc. In FIG. 7,one waveform corresponds to a signal retrieved per pit. Press CDs orCD-R discs are susceptible to level degradation of a high-frequencysignal of, e.g., 3T to 4T. FIG. 8 shows RF reproduction waveforms withthe high-frequency signal degraded due to bad recording conditions ofthe CD-R disc, environmental reasons, etc.

FIG. 9 shows RF reproduction waveforms obtained by performinghigh-frequency compensation on the RF waveforms shown in FIG. 8 usingthe high-frequency compensation unit 11-a shown in FIG. 5. While thewaveforms shown in FIG. 8 contain multiple errors during signalprocessing, the improved waveforms shown in FIG. 9 contain substantiallyno errors.

FIG. 10 shows an example circuit structure of the low-frequencycompensation unit 11-b. In the example shown in FIG. 10, thelow-frequency compensation unit 11-c is configured using a common-baseamplifier. FIG. 11 shows the gain-frequency characteristic of thelow-frequency compensation unit 11-c. As can be seen from FIG. 11, thegain in the low-frequency band, or the low-frequency gain, is higherthan that in the high-frequency band. Thus, the level degradation of thelow-frequency signal of, e.g., 10T to 11T, is compensated for.

FIG. 12 shows normal RF reproduction waveforms of a CD-RW disc. In FIG.12, one waveform corresponds to a signal retrieved per pit. A CD-RW discis susceptible to level degradation of a low-frequency signal of, e.g.,10T to 11T. FIG. 13 shows RF reproduction waveforms with thelow-frequency signal degraded due to bad recording conditions of theCD-RW disc, environmental reasons, etc.

FIG. 14 shows RF reproduction waveform obtained by performinghigh-frequency compensation on the RF waveforms shown in FIG. 13 usingthe high-frequency compensation unit 11-a shown in FIG. 5. As can beseen from FIG. 14, if high-frequency compensation is performed on an RFreproduction waveform with degradation in the low-frequency region,signal errors are not corrected but are further degraded.

FIG. 15 shows RF reproduction waveforms obtained by performinglow-frequency compensation on the RF waveforms shown in FIG. 13 usingthe low-frequency compensation unit 11-c shown in FIG. 10. While thewaveforms shown in FIG. 13 contain multiple errors during signalprocessing, the improved waveforms shown in FIG. 15 containssubstantially no errors. Since CD-RW discs are not susceptible todegradation of a high-frequency signal unlike CD-R discs, it iseffective for such CD-RW discs to perform compensation using thelow-frequency compensation circuit shown in FIG. 10.

The high-frequency compensation circuit and the low-frequencycompensation circuit are not limited to those shown in FIGS. 5 and 10,and multiple modification and variations are conceivable.

While a specific embodiment of the present invention has been describedin detail, it is obvious that a variety of modifications or alternativesmay be made to the specific embodiment by those skilled in the artwithout departing from the scope of the present invention.

An embodiment of the present invention has been described in the contextof, as a digital removable medium, a CD that allows data to be recordedas high-density pits onto a resin disc and that allows the data to beread based on a change in the intensity of the reflected light from thesurface state, such as a pattern of pitted and unpitted areas, on arecording surface of the CD that is irradiated with a laser beam.Although the RF reproduction waveform is degraded in different bandsdepending on the CD format, appropriate frequency-band compensation forthe RF reproduction waveform depending on the format allows the qualityof an RF reproduction signal to be improved for any medium type.

However, the present invention is not limited to this embodiment. Forexample, according to an embodiment of the present invention, a digitalversatile disc (DVD) that is irradiated with a laser beam to retrievedata in a similar manner to CDs by detecting the reflected light of thelaser beam on the disc surface also allows the quality of an RFreproduction signal to be improved for any medium type by implementingappropriate frequency-band compensation for the RF reproduction waveformdepending on the format.

The present invention has been disclosed as an exemplary embodiment, andthe disclosure should not be construed as the restrictive one. Referenceshould be made to the appended claims for delineation of the scope ofthe present invention.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alternatives may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. A disc playback apparatus for reproducing information by demodulatingand decoding a radio-frequency signal obtained by receiving reflectedlight of a beam applied to a recording surface of a removable disc whenthe removable disc is loaded in the disc playback apparatus, theapparatus comprising: disc format determining means for determining theformat of the loaded disc; a high-frequency compensation unit adapted tocompensate for a high-frequency component of the radio-frequency signal;a low-frequency compensation unit adapted to compensate for alow-frequency component of the radio-frequency signal; a control unitadapted to determine which of the high-frequency compensation unit orthe low-frequency compensation unit is to be used to compensate for afrequency characteristic of the radio-frequency signal based on thedetermined disc format; and signal quality determining means fordetermining whether or not the quality of the radio-frequency signal isdegraded, wherein the control unit compensates for the frequencycharacteristic of the radio-frequency signal using the high-frequencycompensation unit or the low-frequency compensation unit in response toa degradation in the quality of the radio-frequency signal that isdetermined by the signal quality determining means.
 2. The disc playbackapparatus according to claim 1, wherein the disc format determining unitdetermines the format of the disc based on an intensity of the reflectedlight from the recording surface of the loaded disc.
 3. The discplayback apparatus according to claim 1, wherein the disc playbackapparatus is compatible with discs of multiple compact-disc formats, andthe control unit determines that the high-frequency compensation unit isto be used to compensate for the frequency characteristic of theradio-frequency signal when a read only memory compact disc or arecordable compact disc is loaded in the disc playback apparatus, anddetermines that the low-frequency compensation unit is to be used tocompensate for the frequency characteristic of the radio-frequencysignal when a recordable erasable compact disc is loaded in the discplayback apparatus.
 4. The disc playback apparatus according to claim 1,wherein the signal quality determining means counts C1 errors in thedemodulated and decoded radio-frequency signal, and determines thequality of the radio-frequency signal based on the counted C1 errors. 5.A disc playback method for reproducing information by demodulating anddecoding a radio-frequency signal obtained by receiving reflected lightof a beam applied to a recording surface of a disc, the methodcomprising the steps of: determining the format of the disc; determiningwhether or not compensation for a frequency characteristic of theradio-frequency signal is necessary; and performing high-frequencycompensation or low-frequency compensation on the radio-frequency signaldepending on the format of the disc when the compensation is necessary,wherein the step of determining whether or not compensation for afrequency characteristic of the radio-frequency signal is necessarycounts C1 errors in the demodulated and decoded radio-frequency signal,and determines whether or not the compensation is necessary based on thecounted C1 error.
 6. The disc playback method according to claim 5,wherein the step of determining the format determines the format of thedisc based on an intensity of the reflected light from the recordingsurface of the disc.
 7. The disc playback method according to claim 5,wherein the step of performing high-frequency compensation orlow-frequency compensation performs high-frequency compensation when thedisc is a read only memory compact disc or a recordable compact disc,and performs low-frequency compensation when the disc is a recordableerasable compact disc.
 8. A disc playback apparatus for reproducinginformation by demodulating and decoding a radio-frequency signalobtained by receiving reflected light of a beam applied to a recordingsurface of a removable disc when the removable disc is loaded in thedisc playback apparatus, the apparatus comprising: a disc formatdetermining unit adapted to determine the format of the loaded disc; ahigh-frequency compensation unit adapted to compensate for ahigh-frequency component of the radio-frequency signal; a low-frequencycompensation unit adapted to compensate for a low-frequency component ofthe radio-frequency signal; and a control unit adapted to determinewhich of the high-frequency compensation unit or the low-frequencycompensation unit is to be used to compensate for a frequencycharacteristic of the radio-frequency signal based on the determineddisc format, wherein the control unit determines whether or not thequality of the radio-frequency signal is degraded, wherein the controlunit compensates for the frequency characteristic of the radio-frequencysignal using the high-frequency compensation unit or the low-frequencycompensation unit in response to a degradation in the quality of theradio-frequency signal that is determined by the signal qualitydetermining means.